1. Field of The Invention
The present invention relates to a method for partially hydrogenating a monocyclic aromatic hydrocarbon to obtain a cycloolefin. More particularly, the present invention is concerned with a method for partially hydrogenating a monocyclic aromatic hydrocarbon to obtain a cycloolefin, in which the hydrogenation reaction is conducted in a reaction system comprising a continuous aqueous phase having a particulate hydrogenation catalyst suspended therein and comprised mainly of metallic ruthenium, an oil phase containing a monocyclic aromatic hydrocarbon and a gaseous phase comprising hydrogen gas, while applying a shearing force to the reaction system at a specific maximum shear rate. By the method of the present invention, not only can a cycloolefin be produced at high selectivity and in high yield, but also the catalytic activity can be stably maintained at a high level for a prolonged period of time.
2. Discussion of Related Art
Cycloolefins are of high commercial value as intermediates for the manufacture of organic chemical products and are particularly important as materials for the production of polyamides and lysines.
As a method for partially hydrogenating a monocyclic aromatic hydrocarbon to produce a cycloolefin, there has conventionally been proposed a method in which use is made of water and a particulate hydrogenation catalyst comprised mainly of metallic ruthenium (see Japanese Patent Application Laid-Open Specification Nos. 61-50930, 62-45544, 62-81332, 62-205037 and 63-17834; these Japanese patent documents correspond to U.S. Pat. No. 4,734,536).
For example, Japanese Patent Application Laid-Open Specification No. 63-17834 mentioned above discloses a working Example in which partial hydrogenation of a monocyclic aromatic hydrocarbon is conducted using a particulate hydrogenation catalyst comprised mainly of metallic ruthenium having an average crystallite diameter of 200 .ANG. or less, a zinc compound as a promoter, and zirconium oxide or hafnium oxide as an additive. In this working Example, an autoclave having an agitator provided therein is charged with water, the above-mentioned catalyst, promoter and additive, and an oil containing benzene and then, the resultant mixture is acidified with a customary acid and the reaction is conducted at a temperature of between 25.degree. and 250.degree. C. under a hydrogen partial pressure of between 5 and 150 kg/cm.sup.2 for a period of time of from several minutes to 2 hours under high-speed agitation while supplying hydrogen gas, to thereby produce cyclohexene. Then, the produced cyclohexene is isolated from the oil phase.
The reaction system used in the above-mentioned conventional method is a heterogeneous system comprised of an oil phase containing a monocyclic aromatic hydrocarbon, an aqueous phase comprising water, a solid phase comprising a particulate hydrogenation catalyst suspended in the aqueous phase, and a gaseous phase comprising hydrogen gas being blown into the reaction system. In the conventional method using such a reaction system, difficult problems have frequently been encountered. Illustratively stated, the conventional method has problems in that when the oil-water mixing and/or gas-liquid mixing is insufficient, the activity of the hydrogenation catalyst in the reaction system cannot be fully exerted and therefore the reaction yield becomes considerably low. On the other hand, when the gas-liquid mixing is too vigorous, not only is the activity of the hydrogenation catalyst rapidly lowered during the reaction, but also the separation between the oil phase and the aqueous phase for obtaining the produced cyclohexene contained in the oil phase needs a long period of time, thus requiring a large stationary zone.
Thus, the conventional method for partially hydrogenating a monocyclic aromatic hydrocarbon has been unsatisfactory not only in that the desired cycloolefin cannot be stably obtained in high yield, but also in that the separation of the oil phase from the aqueous phase after the reaction cannot be efficiently conducted. Therefore, the conventional method cannot be satisfactorily put to practical use.